Variable valve timing system for an internal combustion engine

ABSTRACT

The invention describes a rocker arm displacement system for variable valve timing in internal combustion engines. More specifically, the system provides a rocker arm displacement system including a rocker arm having a pivot, a cam-contacting portion and a valve stem contacting portion and a linear displacement system for linear displacement of the cam-contacting portion with respect to a variable profile cam.

RELATED APPLICATIONS

[0001] This application is related to Canadian Patent Applications2,257,437 and 2,315,595, and to Applicant's co-pending PCT applicationto “Improvements in Cam-Contacting Devices” filed Aug. 7, 2001, all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention describes a rocker arm displacement system forvariable valve timing in internal combustion engines. More specifically,the system provides a rocker arm displacement system including a rockerarm having a pivot, a cam-contacting portion and a valve stem contactingportion and a linear displacement system for linear displacement of thecam-contacting portion with respect to a variable profile cam.

BACKGROUND OF THE INVENTION

[0003] The design of an internal combustion engine requires numeroustrade-offs between conflicting design or performance parametersparticularly with respect to the design of cam profiles and their effecton valve timing.

[0004] For example, in the design of an engine, a designer may wish tominimize exhaust emissions and provide increased fuel economy withoutcompromising satisfactory engine performance. In the past, the design ofsuch an engine would be limited by these conflicting parameters leadingthe designer to compromise with the design to achieve a balance betweenthe parameters. As a result, designers have been forced to focus on aprimary performance goal (such as lower emissions) which may be to thedetriment of desired engine performance (such as torque or idlestability). These compromises are essentially caused by the lack of thedesigner's ability to incorporate breathability into the engine-that is,an optimal intake of fuel and air and the exhaust of spent gases aftercombustion at different operating speeds.

[0005] The timing of air intake and exhaust (breathing) is controlled bythe shape and phase angle of cams. To optimize breathing, enginesrequire different valve timing at different speeds. When rpm increases,the duration of intake and exhaust stroke decreases with the result thatcombustion air cannot enter the combustion chamber fast enough andexhaust gases similarly cannot leave the combustion chamber fast enough.The best solution to this problem is to open the inlet valves earlierand close the exhaust valves later. That is, the overlapping betweenintake period and exhaust period should be increased as rpm increases.

[0006] Furthermore, lift and duration of valve opening also affectsbreathability. At high speed, higher lift quickens air intake andexhaust, However, at lower speed such lift will generate counter effectslike deteriorating the fuel and air mixing process resulting in adecrease in output or even misfire. Therefore the lift and durationshould also be variable according to engine speed.

[0007] With fixed cam engines, a single cam profile is used with theresult that engineers choose a single timing sequence (or cam profile)which is usually a compromise based on the desired generalcharacteristics of the vehicle. For example, a van may adopt lessoverlapping for the benefits of low speed output whereas a racing enginemay adopt considerable overlapping for high-speed power. An ordinarysedan may adopt valve timing optimised for mid-range rpm so that boththe low speed drivability and high-speed output will not be sacrificedto a significant extent. Similarly, lift and duration is determined fora limited rpm range. In other words, the valve timing is optimized for alimited speed range.

[0008] Variable valve timing provides a solution to the above problemsby enabling the adjustment of valve timing as rpm changes with theresult that power and torque are optimized across a wide rpm band.

[0009] Several major manufacturers have designed and implemented variousforms of variable valve timing systems, For example, Honda hassuccessfully implemented its VTEC system (Valve Timing ElectronicControl) which essentially uses two (or three) different set of camswhich are specifically optimized for different rpm ranges andmechanically actuated based on engine speed. The VTEC system does notallow a continuous change of timing but does provide two (or three)distinct phases of performance. The cam-changing system of the VTECengine is mechanically complex.

[0010] Another system is the Toyota VVTL-I which provides both variablecamphasing and lift and duration. This system does not providecontinuous variation of lift and duration but instead utilizes a 2-stagevariable lift design.

[0011] Other proposed designs include the use of continuously variablecam profiles in which a camshaft having a three-dimensional cam profileis linearly displaced across a cam follower. Such systems allow lift,duration and degreeing to be addressed with a 3 dimensional cam profile.However, such systems have not been successful as a result of thefailure of components of cam followers brought on by contact stresses atthe cam follower/cam interface. That is, the cam followers of a fixedprofile cam minimize the compressive loads of the cam follower on thecam by distributing the compressive load across a wider area. In acontinuously variable system, a fine contact point is required in orderto optimize the resolution of the cam profile being used. This finecontact point, for a given valve spring pressure, results in higherinterface pressures as compared to the standard contacting devices.These higher pressures will cause cam followers, implemented usingconventional materials, to fail.

[0012] Thus, while various variable timing systems have been proposed,there remain several problems with respect to the implementation of thetechnology to its theoretical potential. That is, the currentstate-of-the-art as implemented by Honda, Toyota and other major enginemanufacturers provides variable valve timing systems which aremechanically complex and which do not provide true continuously variabletiming with respect to lift, duration and phasing. Furthermore, variablevalve timing systems which propose continuously variable lift andduration have been unsuccessful as a result of cam follower failure.

[0013] Accordingly, there has been a need for variable valve timingsystems which provide the ability to adjust lift, duration and degreeingwith a relatively simple mechanical system which does not require thecomplex mechanical interplay of many mechanical components. Inparticular, there has been a need for a system which provides for a finecam follower/cam interface and which can survive the high compressiveloads at this interface. Further still, there has been a need for avariable valve timing system which can be retrofit to existing vehiclesby replacing a rocker arm/fixed cam system with a linear displaceablerocker arm/variable profile cam system or factory installed into newengines. Further still, there has been a need for systems which willsignificantly reduce fuel consumption and emissions particularly duringidling without affecting vehicle performance.

[0014] In particular, and with reference to applicant's copendingCanadian application 2,315,595, a solution to cam follower failure isprovided in which ceramic materials are used as cam-contacting deviceswhere ceramic materials have been demonstrated to overcome the problemsof cam follower failure. More particularly, the use of silicon nitridehas been shown to be particularly effective as a material for use as acam-contacting surface in an internal combustion engine.

SUMMARY OF THE INVENTION

[0015] In accordance with one embodiment of the invention, there isprovided a rocker arm displacement system for linear displacement of arocker arm having a cam contacting portion with respect to a rotating,variable-profile cam comprising a linear displacement system operativelyconnected to a rocker arm.

[0016] In accordance with another embodiment, there is provided a rockerarm displacement system comprising:

[0017] a rocker arm having a pivot, a cam contacting portion and a valvestem contacting portion;

[0018] a linear displacement system for linear displacement of the camcontacting portion with respect to a variable profile cam

[0019] In further embodiments, the cam-contacting device has improvedthermal dissipation properties including a coefficient of thermalexpansion less than 3×10⁻⁶/degree Celsius.

[0020] In other embodiments, the cam contacting system is a ball bearingand a bearing race and support and the coefficient of thermal expansionof the ball bearing is less than the coefficient of thermal expansion ofthe bearing race and support.

[0021] Further still, in preferred embodiments, the cam contactingsystem is ceramic selected from any one of silicon nitride (includingCERALLOY 147-31E, 147-31N, 147-1E, or 147-1) or silicon carbide.

[0022] In other embodiments, it is preferred that the cam contactingdevices is selected from any one of a radiused wheel, a ball bearing ora semi-spherical surface and wherein the cam contacting surface isselected from any one of CERALLOY 147-31E, 147-31N, 147-1E, or 147-1.The valve stem contacting portion may also be selected from thesematerials.

[0023] In a further still embodiment, the linear displacement systemincludes a sliding block operatively connected to a linear displacementcylinder and the sliding block pivotally retains the rocker arm.

[0024] It is also preferred that a lash adjustment system is providedwithin the sliding block, rocker arm or valve stem. In a more specificembodiment, the lash adjustment system is hydraulic and includes apiston within the sliding block for setting the range of motion of therocker arm within the sliding block.

[0025] In a further embodiment, the rocker arm is pivotally connected tothe valve stem.

[0026] In a more specific embodiment, the invention provides a rockerarm displacement system comprising:

[0027] a rocker arm having a pivot, a cam contacting system and a valvestem contacting system, the cam contacting system a silicon nitride ballbearing rotatably retained within the rocker arm;

[0028] a silicon nitride valve stem tip for operative engagement withthe valve stem contacting system; and

[0029] a sliding block operatively connected to a linear displacementcylinder for linear displacement of the cam contacting portion withrespect to the variable profile cam wherein the sliding block pivotallyretains the rocker arm and wherein the sliding block includes ahydraulic lash adjustment system.

[0030] In another embodiment, the invention provides a method ofretrofitting an internal combustion engine with a rocker armdisplacement system as in claim 1, the internal combustion engine havinga valve cover, valve stems, a rocker arm assembly and a camshaftcomprising the steps of:

[0031] a) removing the valve cover to expose the valve stems, rocker armassembly and camshaft;

[0032] b) removing the camshaft and installing a variable profilecamshaft;

[0033] c) installing a rocker arm displacement system having a rockerarm, a cam contacting portion, a valve stem contacting portion and alinear displacement system.

[0034] Further still, the invention provides an internal combustionengine characterized by a rocker arm displacement system for lineardisplacement of a rocker arm with respect to a camshaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The invention is described with reference to the followingdrawings in which:

[0036]FIGS. 1A, 1B and 1C are schematic drawings of the geometric designof three embodiments of a rocker arm displacement system showing therelative positions of the valve stem/spring, pivot and cams with respectto a rocker arm;

[0037]FIGS. 2A and 2B are forward and rear perspective views of a rockerarm displacement system for a variable valve timing system for anoverhead cam engine in accordance with one embodiment of the invention;

[0038]FIG. 2C is a partial cross-sectional view of a rocker armdisplacement system for a variable valve timing system for an overheadcam engine in accordance with one embodiment of the invention showingdetails of a linear displacement cylinder;

[0039]FIGS. 3A and 3B are forward and end perspective views of a rockerarm displacement system for a variable valve timing system for anoverhead cam engine in accordance with another embodiment of theinvention;

[0040]FIG. 3C is a partial cross-sectional view of a rocker armdisplacement system for a variable valve timing system for an overheadcam engine in accordance with another embodiment of the inventionshowing details of a linear displacement cylinder and sliding block;and,

[0041]FIG. 4 is a schematic diagram of a further embodiment of a rockerarm displacement system for variable valve timing system for an overheadcam engine in accordance with a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0042] System Overview

[0043] With reference to the Figures, a rocker arm displacement system10 for variable valve timing is described.

[0044] The system generally includes a rocker arm (R) in operativeconnection with a valve stem (V)/spring (Vs), a cam contacting device(CCD) contacting a variable profile cam shaft (C) and a lineardisplacement system (LDS) for linear displacement of the rocker arm withrespect to the cam shaft.

[0045] Different embodiments of the system are schematically illustratedin FIGS. 1A, 1B and 1C wherein options for the geometric layout of therocker arm (R), camshaft (C), valve stem (V) /spring (Vs, with ↑ showingthe direction of spring force) are shown with respect to a rocker armpivot (P). FIG. 1A shows a rocker arm having a centrally located pivotwith both the valve stem/spring and camshaft on the same side of therocker arm. FIG. 1B shows a rocker arm with the pivot at one end with acentrally positioned camshaft and with the valve stem/spring on theopposite side and end of the rocker arm. FIG. 1C shows an embodimentwith the pivot at one end with a centrally positioned valve stem/springand with the camshaft on the opposite side and end of the rocker arm.

[0046]FIGS. 2A, 2B and 2C show perspective and cross-sectional views ofthe embodiment of FIG. 1A. The camshaft includes variable profile lobes20 on a rotating camshaft C. Rocker arms R for each cam are mounted on arocker shaft 22 at the rocker arm pivot and include cam followers orcam-contacting devices 24 biased against each respective variableprofile lobes by valve springs Vs acting on the opposite end of therocker arm. A linear displacement system (LDS) is operatively connectedto the rocker shaft and includes a hydraulic cylinder 26 for lineardisplacement of the rocker shaft 22 and, hence, linear displacement ofthe cam followers 24 with respect to the variable profile lobes 20.

[0047] As shown in FIG. 2A, the cam followers 24 are spherical ballbearings rotatably retained within bearing races within the rocker arm.As described in applicant's copending application, Canadian application2,315,595, the spherical bearing provides a fine contact point withrespect to the cam lobe thereby permitting continuous setting of thevalve timing between the end limits of the cam 20.

[0048] With reference to FIG. 2B, the opposite ends of the rocker armscontact the valve stems and include a valve stem contacting device suchas valve stem bearing 28 rotatably retained within a tip ball retainer30 (as shown in FIG. 2b). Alternatively, valve stem bearing may berotatably retained within the rocker arm (not shown). In addition, therocker arm will preferably include a hydraulic lash adjustment system 32enabling adjustment of the clearance between the rocker arm and valvestem.

[0049] As with the cam follower 24 and camshaft, the valve stem end ofthe rocker arm is displaced with respect to the valve stem when actuatedby the LDS.

[0050] With reference to FIG. 2C, a partial cutaway of the actuationcylinder of the linear displacement system is shown having dualhydraulic oil ports 34. In this embodiment, the actuation cylinder 26may be positively positioned at a full range of positions across thewidth of the cam as may be determined by an appropriate controlalgorithm, such as one where the linear position of the rocker arms isproportional to engine rpm.

[0051] With reference to FIGS. 3A, 3B and 3C, a second embodiment of thesystem corresponding to that shown schematically in FIG. 1B isdescribed. In this embodiment, the pivot is located at one end of therocker arm with the cam followers located in the middle of the rockerarm and valve stem/spring located at the opposite end.

[0052] As shown in FIG. 3A, the linear displacement system includes asliding block 40 which pivotally retains the rocker arm. In thisembodiment, the sliding block permits pivotal up and down motion of therocker arm but prevents side-to-side motion of the rocker arm within thesliding block. As shown, the sliding block is retained within aretaining system 42 allowing linear displacement of the sliding blockunder the control of the linear displacement cylinder 26.

[0053] The sliding block may also contain a lash adjustment system 32 apermitting adjustment of the clearance between the valve stem and rockerarm. The lash adjustment system may be hydraulically controlled whereinthe sliding block includes hydraulic pistons 32 b (as shown in cutawayin FIG. 3C) for setting the range of motion of the rocker arm.

[0054] As in the FIG. 2 embodiment, the valve stem includes a valve stemcontacting device such as a valve stem bearing 28 rotatably retainedwithin a tip ball retainer 30 (as shown in FIG. 3B).

[0055] In another embodiment, as shown schematically in FIG. 4 (planview), the rocker arm is pivoted about a second axis P2 orthogonal tothe main pivot axis P of the rocker arm. That is, the rocker arm doesnot include a tip bearing or roller but rather is pivotally connected tothe valve stem and is pivotally connected to the LDS thereby allowingrotational movement of the rocker arm with respect to the valve stem.

[0056] Cam Contacting Device

[0057] The cam-contacting device may be embodied in different formsincluding a ball bearing, a roller, a tapered wheel and a half sphere asdescribed in applicant's copending applications. The cam-contactingdevice will preferably provide a fine contact point between thecam-contacting device and cam so as to provide for continuousvariability of the cam profile between the different ends of the cam.Furthermore, the cam-contacting device is preferably a ceramic materialsuch as silicon nitride, which provides improved thermal propertiesbetween the cam contacting device and the cam to accommodate the highcontact stresses.

[0058] Valve Stem Contacting System

[0059] The valve stem contacting system may be embodied in differentforms to achieve the result of valve actuation while permitting linearor pivotal movement of the rocker arm with respect to the valve stem.Accordingly, the valve stem contacting system may include a bearingsystem or other contact system located on either the rocker arm or valvestem or between the rocker arm and valve stem. The bearing system may bea ball bearing or roller bearing. Other contact systems may include flator curved surfaces engageable with one another and linearly displaceablewith respect to one another. The valve stem contacting system preferablyutilizes a ceramic material such as silicon nitride, which providesimproved thermal properties between the rocker arm and the valve stem toaccommodate the high contact stresses.

[0060] As shown in FIG. 4, the rocker arm may also be pivotallyconnected to the valve stem.

[0061] Linear Displacement System

[0062] The linear displacement system may be actuated by a hydrauliccylinder as described above or other linear motion systems as are knownin the art including electromechanical systems.

[0063] Retrofitability

[0064] The rocker arm displacement system as described is readilyretrofit to existing overhead cam engines. In order to retrofit thesystem to an existing overhead cam engine, the following generalprocedure would be followed. Initially, the valve cover would be removedto expose the existing rocker arm and camshaft assembly. The existingfixed profile camshaft would be removed and replaced with a variableprofile camshaft. The existing rocker arm assembly would similarly beremoved. Any required modification to the valve stem tips would becompleted for the particular design of the valve stem contacting system.A new rocker arm displacement assembly would be installed between thecamshaft and valve stems, likely requiring the addition of an adaptiveplate to permit linear movement of the new rocker arm assembly.Modification to the valve cover will likely be required for the lineardisplacement system to extend through the valve cover, if required.Alternatively, a specifically designed valve cover assembly may beutilized to accommodate the linear displacement system. The lineardisplacement system would be connected to an appropriate control systemsuch as an electro-hydraulic, electromechanical or electrical system.

[0065] It is also envisaged that specific cylinder heads be designed toincorporate a rocker arm displacement system such that retrofitting isnot required but rather is factory installed.

1. A rocker arm displacement system for linear displacement of a rockerarm having a cam contacting portion with respect to a rotating,variable-profile cam comprising a linear displacement system operativelyconnected to a rocker arm.
 2. A rocker arm displacement systemcomprising: a rocker arm having a pivot, a cam contacting portion and avalve stem contacting portion; a linear displacement system for lineardisplacement of the cam contacting portion with respect to a variableprofile cam
 3. A rocker arm displacement system as in claim 2 whereinthe cam-contacting device has improved thermal dissipation properties.4. A rocker arm displacement system as in claim 2 wherein the camcontacting portion has a coefficient of thermal expansion less than3×10⁻⁶/degree Celsius.
 5. A rocker arm displacement system as in claim 2wherein the cam contacting system is a ball bearing and a bearing raceand support and wherein the coefficient of thermal expansion of the ballbearing is less than the coefficient of thermal expansion of the bearingrace and support.
 6. A rocker arm displacement system as in claim 2wherein the cam contacting system is ceramic.
 7. A rocker armdisplacement system as in claim 2 wherein the cam contacting system isany one of silicon nitride or silicon carbide.
 8. A rocker armdisplacement system as in claim 2 wherein the cam contacting system isselected from any one of CERALLOY 147-31E, 147-3 IN, 147-1E, or 147-1.9. A rocker arm displacement system as in claim 2 wherein the camcontacting system is selected from any one of a radiused wheel, a ballbearing or a semi-spherical surface and wherein the cam contactingsurface is selected from any one of CERALLOY 147-31E, 147-31N, 147-1E,or 147-1.
 10. A rocker arm displacement system as in claim 2 wherein thevalve stem contacting portion is silicon nitride.
 11. A rocker armdisplacement system as in claim 2 wherein the linear displacement systemincludes a sliding block operatively connected to a linear displacementcylinder and the sliding block pivotally retains the rocker arm.
 12. Arocker arm displacement system as in claim 12 wherein the sliding blockincludes a hydraulic lash adjustment system.
 13. A rocker armdisplacement system as in claim 13 wherein the hydraulic lash adjustmentsystem includes a piston within the sliding block for setting the rangeof motion of the rocker arm within the sliding block.
 14. A rocker armdisplacement system as in claim 2 wherein the valve stem includes a tipbearing for engagement with the rocker arm.
 15. A rocker armdisplacement system as in claim 2 wherein the rocker arm has a lashadjustment system adjacent the valve stem end of the rocker arm.
 16. Arocker arm displacement system as in claim 2 wherein the rocker arm hasa tip bearing for engagement with the valve stem.
 17. A rocker armdisplacement system as in claim 2 wherein the cam contacting portion isa ball bearing for engagement with the cam.
 18. A rocker armdisplacement system as in claim 2 wherein the cam contacting portion isa tapered wheel for engagement with the cam.
 19. A rocker armdisplacement system as in claim 2 wherein the rocker arm is pivotallyconnected to the valve stem.
 20. A rocker arm displacement system as inclaim 2 wherein the rocker arm pivot is located centrally to the valvestem end and cam contacting end of the rocker arm.
 21. A rocker armdisplacement system as in claim 2 wherein the rocker arm pivot islocated at a first end of the rocker arm.
 22. A rocker arm displacementsystem as in claim 5 wherein the cam contacting system is any one of aradiused wheel, a ball bearing or a semi-spherical surface and is asilicon nitride selected from any one of CERALLOY 147-31E, 147-31N,147-1E, or 147-1 or a silicon carbide.
 23. A rocker arm displacementsystem as in claim 22 wherein the valve stem contacting portion issilicon nitride.
 24. A rocker arm displacement system as in claim 23wherein the linear displacement system includes a sliding blockoperatively connected to a linear displacement cylinder and the slidingblock pivotally retains the rocker arm.
 25. A rocker arm displacementsystem as in claim 24 wherein the sliding block includes a hydrauliclash adjustment system.
 26. A rocker arm displacement system as in claim25 wherein the hydraulic lash adjustment system includes a piston withinthe sliding block for setting the range of motion of the rocker armwithin the sliding block.
 27. A rocker arm displacement system as inclaim 26 wherein the valve stem includes a tip bearing for engagementwith the rocker arm.
 28. A rocker arm displacement system as in claim 27wherein the rocker arm has a lash adjustment system adjacent the valvestem end of the rocker arm.
 29. A rocker arm displacement system as inclaim 28 wherein the rocker arm has a tip bearing for engagement withthe valve stem.
 30. A rocker arm displacement system comprising: arocker arm having a pivot, a cam contacting system and a valve stemcontacting system, the cam contacting system a silicon nitride ballbearing rotatably retained within the rocker arm; a silicon nitridevalve stem tip for operative engagement with the valve stem contactingsystem; and a sliding block operatively connected to a lineardisplacement cylinder for linear displacement of the cam contactingportion with respect to the variable profile cam wherein the slidingblock pivotally retains the rocker arm and wherein the sliding blockincludes a hydraulic lash adjustment system.
 31. A method ofretrofitting an internal combustion engine with a rocker armdisplacement system as in claim 1, the internal combustion engine havinga valve cover, valve stems, a rocker arm assembly and a camshaftcomprising the steps of: a) removing the valve cover to expose the valvestems, rocker arm assembly and camshaft; b) removing the camshaft andinstalling a variable profile camshaft; c) installing a rocker armdisplacement system having a rocker arm, a cam contacting portion, avalve stem contacting portion and a linear displacement system.
 32. Aninternal combustion engine characterized by a rocker arm displacementsystem for linear displacement of a rocker arm with respect to acamshaft.